Batch exposure-type (e.g., stepper and the like) projection exposure apparatus are used to manufacture semiconductor devices and the like. Such apparatus may be of the scanning exposure-type (step-and-scan type), where each exposure field is scanned during exposure, or step-and-repeat type, where each exposure field is exposed in a single static exposure. A projection exposure apparatus as used in semiconductor manufacturing, for example, transfers the image of a pattern on a reticle, which is used as a mask, through a projection optical system and onto a wafer (or glass plate or the like) coated with a light-sensitive medium, such as photoresist. With the increasing miniaturization of the patterns of semiconductor integrated circuits and the like, there have been increasing demands to improve the resolving power of projection optical systems incorporated into semiconductor exposure apparatuses. The resolving power of the projection optical system can be improved by either shortening the exposure wavelength or increasing the image-side numerical aperture (NA).
With regard to shortening the exposure wavelength, the wavelength of the light sources of the illumination systems used in exposure apparatuses have progressively evolved from the longer UV wavelengths, such as the mercury lamp g-line (436 nm wavelength) and the i-line (365 nm wavelength), down to the shorter (i.e., "deep") UV wavelengths associated with excimer lasers, such as the KrF laser line (248 nm wavelength) and the ArF laser line (193 nm wavelength). Therefore, in accordance with this trend projection optical systems capable of transmitting exposure light at the deep UV wavelengths are being developed.
In addition to the increasing demands on resolution, there have also been increasing demands to decrease the amount of image distortion of the projection optical systems. Image distortion as a whole includes several contributing factors, such as distortion inherent in the projection optical system itself, distortion due to warping of the wafer upon which the circuit pattern is printed, and distortion due to warping of the reticle on which a circuit pattern and the like is drawn. To reduce the effect of image distortion due to warping of the wafer, imagewise telecentric projection optical systems have been developed. In such systems, the exit pupil is located at infinity objectwise of the projection optical system.
Also, objectwise telecentric optical systems have been employed, wherein the entrance pupil of the projection optical system is located at infinity objectwise of the projection optical system. This reduces image distortion due to warping of the reticle. Such projection optical systems are disclosed in Japanese Patent Application Kokai No. Sho 63-118115, Japanese Patent Application Kokai No. Hei 4-157412 and Japanese Patent Application Kokai No. Hei 5-173065.
In addition, there have been demands for being able to select and adjust the NA to be more ideally suited for printing particular types of patterns on the reticle, as well as to account for other manufacturing conditions. In particular, there have been demands for the projection optical systems in exposure apparatuses to have a variable aperture stop whose size can be varied to change the NA of the projection optical system.
As described above, it is desirable to make the projection optical system both imagewise and objectwise telecentric to reduce the effects of both wafer warping and reticle warping on image distortion. Therefore, as disclosed in the abovementioned patent applications, projection optical systems have been developed that are telecentric both in the object space and the image space, i.e., so-called "doubly telecentric" projection optical systems. Nevertheless, in prior art doubly telecentric projection optical systems, it has proven difficult to make the NA sufficiently large while simultaneously reducing the various aberrations over a large field. In particular, in the prior art systems, distortion correction is inadequate.
Moreover, in the prior art projection optical systems, if a variable aperture stop is provided to vary the NA of the projection optical system, vignetting occurs at the peripheral part of the exposure field when the aperture stop size is changed, due to spherical aberration at the pupil. Consequently, uniformity of illumination suffers in the peripheral part of the exposure field. In addition, telecentricity degrades when the numerical aperture is varied, and there is also the problem that the exposure field size cannot be increased.